Colorectal cancer (CRC) is the second leading cause of cancer death in the United States. Studies indicate that CRC is the consequence of stepwise changes (mutations) in a series of genes with important cellular functions including tumor suppressor genes (TSGs) and oncogenes. Thus, somatic or germline inactivation of TSGs such as adenomatous polyposis coli (APC), p53, and several genes involved in DNA mismatch repair;and oncogenic activation of KRAS and BRAF are thought to be crucial in the formation of CRC. Moreover, CRC is genetically unstable and this instability arises from two different pathways: chromosomal instability (CIN) and microsatellite instability (MIN). CIN, which accounts for 85% of all CRC, is thought to underlie the aneuploid state observed in CRC. The molecular mechanism that leads to CIN, however, is not well established. Our group previously identified a zinc finger-containing transcription factor called gut-enriched Kruppel-like factor (GKLF;also known as Kruppel-like factor 4 or KLF4), the expression of which is enriched in the terminally differentiated epithelial cells of the gut. KLF4 is an inhibitor of cell proliferation and does so by transcriptionally activating the gene encoding the cyclin-dependent kinase inhibitor, p21(WAF1/Cip1). KLF4 is necessary and sufficient in mediating the inhibitory effect of the tumor suppressor, p53, on the cell cycle following DNA damage, p53-dependent induction of KLF4 leads to a block in both the G1/S and G2/M transitions in the cell cycle and prevents the entry into mitosis. Preliminary data also indicate that KLF4 is required for maintaining stability of centrosomes in cells that sustained DNA damage due to gamma irradiation. Consistent with its growth inhibitory effect, expression of KLF4 is decreased in intestinal tumors derived from the Apc(Min/+) mice, colonic adenomas from FAP patients and sporadic CRC, when compared to their respective normal adjacent mucosa. Moreover, we showed evidence of loss of heterozygosity (LOH) in the KLF4 locus in a subset of sporadic human CRC, suggesting that KLF4 itself is a tumor suppressor gene in CRC. Based on these findings, we hypothesize that KLF4 is an important tumor suppressor in CRC and is involved in maintaining genomic stability. The long-term goal of the proposed project is to understand the molecular mechanisms responsible for the formation of colorectal cancer. In the project period supported by this competitive renewal, we propose 3 Specific Aims: (1) To investigate the role of KLF4 in maintaining centrosome stability following DNA damage, (2) To determine whether KLF4 is involved in maintaining genetic stability following DNA damage, and (3) To investigate the role of KLF4 as a tumor suppressor in vivo in transgenic mice. The completion of these aims will further advance the understanding of the molecular mechanisms of colorectal cancer formation.